1. Field of the Invention
The present invention relates to an optical recording medium on which recording and reproduction are performed by using an optical head which carries a solid immersion lens. In particular, the present invention especially relates to an optical recording medium and an optical recording apparatus for the same which improve the durable performance for sliding movement effected between an optical head which carries a solid immersion lens and the optical recording medium, making it possible to record information at a high density by utilizing evanescent light, and reproduce the recorded information with high C/N.
2. Description of Related Art
Recently, the optical recording apparatus, which is an information-recording apparatus capable of recording a large capacity of data at a high density and quickly reproducing recorded data, is used in response to the development of the information-recording apparatus to conform to the multimedia. The optical recording medium, which is subjected to recording or reproduction by using the optical recording apparatus, includes the read-only disk such as CD and laser disks in which information is permanently stored by stamping the disk surface to have a concave-convex configuration upon production of the disk, the write-once type disk such as CD-R which are capable of recording only once, and the rewritable type disk in which data can be rewritten and erased any number of times by using the magneto-optical recording system or the phase-change recording system. When the information is recorded or reproduced on the optical recording medium by using the optical recording apparatus, a light spot is used, which is obtained by focusing a laser beam up to the diffraction limit by using a lens. The size d of the light spot is represented by d=.lambda./NA provided that the wavelength of the laser is .lambda., and the numerical aperture of the lens is NA ("Foundation and Application of Optical Disk Storage", edited by Yoshito Tsunoda, Incorporated Association of Society of Electronic Information and Communication (1995), p. 65).
In order to record information on the optical recording medium at a higher density, it is necessary to decrease the recording laser spot size so that minute pits and magnetic marks are formed. However, according to the expression described above, in order to decrease the light spot, the laser wavelength (.lambda.) may be decreased, or the numerical aperture (NA) of the lens is increased. The semiconductor laser for performing reproduction on the optical disk used at present has the wavelength which is mainly 780 to 680 nm. A laser of orange color of 650 nm, which is shorter in wavelength than the above, begins to be used, for example, for the digital versatile disk (DVD-ROM). However, the short wavelength laser, which emits green or blue light of a wavelength shorter than the wavelength of the orange laser, is still under development. There is a limit to decrease the spot size by decreasing the laser wavelength.
On the other hand, as shown in FIG. 1, the numerical aperture (NA) of the lens is represented by NA=sin .phi. provided that the focusing half angle of the lens is .phi.. The numerical aperture NA has a value smaller than 1. The lens, which is used at present, has NA of about 0.5. Even if NA=0.9, which approximates to the theoretical limit, is achieved, the laser spot size can be merely reduced to be 1/1.8 at most. On the other hand, if NA is increased, then the depth of focus of the lens system becomes shallow, and a problem arises in that a complicated control system should be used to maintain the focal point on the recording plane. Therefore, it is impossible to excessively increase NA. In the case of an ordinary optical recording apparatus, a lens approximately having NA=0.6 is used at the maximum.
A method for effectively increasing NA of the lens has been suggested, in which a solid immersion lens is used in order to decrease the spot size of the laser beam (Nikkei Electronics, No. 686, pp. 13-14, 1997.4.7). As shown in FIG. 2A, when a hemispherical solid immersion lens is used, and the laser beam is allowed to come perpendicularly into the lens surface, then the equivalent NA of the optical system is represented by n.times.NA provided that the refractive index of the solid immersion lens is n. As shown in FIG. 2B, when a super spherical solid immersion lens is used, and the laser beam is allowed to come so that the focal point is formed on the bottom surface of the super spherical lens, then the equivalent NA is represented by n.sup.2 .times.NA. When the solid immersion lens is made of glass, the refractive index of glass is about 1.8. Therefore, the spot size can be decreased to be 1/1.8 when the hemispherical solid immersion lens is used, and the spot size can be decreased to be 1/3.2 when the super spherical solid immersion lens is used, respectively as compared with the case in which an ordinary objective lens is used.
When the solid immersion lens is used, the evanescent light, which leaks out from the solid immersion lens, can be used to perform recording and reproduction. The attenuation distance of the evanescent light is not more than the wavelength of the light emitted from the light source. Therefore, it is necessary to allow the solid immersion lens to approach the medium so that the solid immersion lens is disposed within the attenuation distance of the evanescent light. For this reason, when the solid immersion lens and the evanescent light are used in combination, it is necessary to use a flying type slider as used for a magnetic head of a fixed type magnetic disk (hard disk). FIG. 3 shows an example of the structure of the optical head for the magneto-optical recording medium based on the use of such a flying type slider. The optical head comprises an objective lens 71, a solid immersion lens 100, and a recording magnetic field-generating coil 104 incorporated into a flying type slider 102.
In the case of an ordinary magneto-optical recording apparatus, the light is radiated onto the recording layer through a transparent substrate of the magneto-optical recording medium. However, in the case of the optical head based on the use of the solid immersion lens, the spacing distance between the solid immersion lens and the optical recording medium is restricted as described above. Therefore, the magneto-optical recording medium adopts a structure in which a reflective layer, a second dielectric layer, a magneto-optical recording layer, and a first dielectric layer are stacked in this order on a substrate. It is necessary for the magneto-optical recording medium to be irradiated with the recording and reproducing light beam from the side opposite to the substrate, i.e., from the side of the first dielectric layer. For example, U.S. Pat. No. 5,202,880 discloses an optical recording medium and a recording and reproducing apparatus of the type in which the recording and reproducing light beam comes from the side opposite to the substrate.
In the case of the ordinary optical disk, in order to protect the recording layer, a protective layer is formed on the side of the recording layer surface opposite to the substrate by applying ultraviolet-curable resin or Si resin curable in the atmospheric air. The protective layer generally has a thickness of several .mu.m to several tens of .mu.m. In the case of the system based on the use of the evanescent light by the aid of the solid immersion lens, it is impossible to form the protective layer made of resin on the second dielectric layer, because the protective layer made of resin is thicker than the attenuation distance of the evanescent light. Therefore, in this system, the recording and reproducing optical head is moved at a position separated by about 100 nm from the first dielectric layer as the uppermost layer, in the same manner as in the fixed type magnetic disk apparatus. For this reason, if the optical head varies its flying posture during the movement, then the optical head contacts with the second dielectric layer, and its surface is scratched in some cases.
The first dielectric layer, which is disposed at the uppermost layer of the magneto-optical recording medium, is formed of a hard material such as silicon nitride, silicon oxide, aluminum nitride, and silicon carbide. The film thickness thereof is 50 to 100 nm which is two to five times thicker than that used in the magnetic disk. The flying height of the recording and reproducing optical head can be 100 to 150 nm which is about two to three times higher than that used in the magnetic disk, when the evanescent light is used by the aid of the solid immersion lens. Therefore, the scratch, which is formed on the surface of the dielectric layer due to irregular sliding movement caused by variation of the posture of the optical head, does not become so deep to arrive at the recording layer. In many cases, the scratch is formed in the traveling direction of the optical head, as a grazed stripe-shaped scratch having a width of about several .mu.m to several hundreds of .mu.m and a depth of about several tens nm. In the case of the optical disk such as an ordinary magneto-optical disk, the laser spot size is about 1 mm at the minimum on the surface of the substrate, because the reproduction is performed through the transparent substrate. Therefore, the scratch of about several .mu.m to several tens of .mu.m formed on the substrate surface scarcely causes problems concerning recording and reproduction. However, the system, which is based on the use of the evanescent light by the aid of the solid immersion lens, utilizes the leakage of the laser beam having been focused up to the diffraction limit. Therefore, a problem arises in that the scratch, which merely has a width of several .mu.m formed on the surface of the dielectric layer, tends to cause any reproduction error due to variation of the amount of reflected light, i.e., the amount of reproducing light, caused by the interference at the edge of the scratch.
Further, there has been also a problem that when recording or reproduction is performed by means of the evanescent light by using the solid immersion lens, then the structure of the optical recording medium should be properly adjusted, and the positional relationship between the solid immersion lens and the optical recording medium should be properly adjusted in accordance therewith, because the attenuation distance of the evanescent light is extremely short as described above.
Moreover, if there is any irregularity on the surface of the optical recording medium, the flying height of the optical head supported by the flying type slider varies depending thereon. However, if the flying height of the optical head varies, any multiple interference of light may occur depending on the flying amount, in the air layer intervening between the surface of the optical recording medium and the light-emitting plane of the optical element installed to the optical head. If such interference is caused, the intensity of reflected light changes. The change in intensity of reflected light causes variation in reproduced signal output from the optical recording medium, possibly resulting in reproduction error.